different visual pigments and therefore provide for color vision. Loss of cone function results in functional blindness; rod vision is not sufficient for normal visual requirements.
The receptive field of an individual photoreceptor is circular. Light in the receptive field hyperpolarizes the photoreceptor cell and cause it to release less neurotransmitter. The receptive fields of photoreceptors and retinal interneurons determine the receptive fields of the retinal ganglion cells onto which their activity converges. The characteristics of the receptive fields of retinal ganglion cells constitute an important step in visual information processing because all the information about visual events that is conveyed to the brain is contained in ganglion cell activity.
The bipolar cell, which receives input from a photoreceptor, can have either of two types of receptive fields, as shown in
Fig. 8.8.
Both are described as having a center-surround organization in which the light that strikes the central region of the receptive field either excites or inhibits the cell, whereas the light that strikes a region that surrounds the central portion has the converse effect. The receptive field with a centrally located excitatory region surrounded by an inhibitory annulus is called an on-center, off-surround receptive field (see
Fig. 8.8A ). Bipolar cells with such a receptive field are described as “on” bipolar cells. The other type of receptive field has an off-center, on-surround arrangement, which characterizes “off” bipolar cells (see
Fig. 8.8F
The center response of a bipolar cell receptive field is due to only the photoreceptors that directly synapse with the bipolar cell. Photoreceptor cells respond to light with hyperpolarization and a decrease in glutamate release and respond to the removal of light with depolarization and increased glutamate release. This implies that the difference in the center responses of “on” and “off ” bipolar cells lies in their response to glutamate. In fact, off-center bipolar 136 SECTION2Berne & Levy Physiology A On-center field 0 Sec 0.5 1.0 1.5 Central spot of light Light Peripheral spot F Off-center field B On-center cell responses C Central illumination D Annular illumination E Diffuse illumination 0 Sec 0.5 1.0 1.5 H Central illumination I Annular illumination J Diffuse illumination G Off-center cell responses
Fig. 8.8 Thereceptivefieldsofon-center(A) andoff-center(F) bipolarcellsand,belowthem,thereceptivefieldsofganglioncellsB throughE andG throughJ towhichtheyareconnected.Ganglioncellresponsestocentralspots(upper recording)andperipheralspots(lower recording)areshowninB and
G.
Alsoshownareresponsestocentral(C andH),surround(D andG),anddiffusewhole-field(E and
J)illuminationintheirreceptivefields.Theganglioncellsandtheon-centerandoff-centerbipolarcellsprovidinginputtotheseganglioncellshavesimilarreceptivefields,butwhereasganglioncellsincreaseordecreasetheirspikefrequency,bipolarcellsdepolarizeorhyperpolarize,withoutgeneratingactionpotentials.(RedrawnfromSquireLRetal[eds].Fundamental Neuroscience. SanDiego,CA:AcademicPress;2002.) cells have ionotropic glutamate receptor channels that open in response to glutamate, and thus they are excited by the removal of light stimuli from the center of their receptive field. In contrast, on-center bipolar cells have metabotropic glutamate receptors that close their channels in response to glutamate. They are depolarized by light on the center of their receptive field, because the reduced release of glutamate by the photoreceptors results in more open metabotropic channels. Thus on-center bipolar cells are excited by light stimulation of the center of their receptive fields.